Metagenomic insight into the distribution of metal resistance genes within cascade reservoir waters: Synergic impacts of geographic variation and anthropogenic pollution

Metal resistance genes (MRGs) are potential bio-indicators to diagnose contamination stress on riverine ecosystems. Within reservoir systems, river damming weakens hydrodynamic condition and enriches metal contaminants. But, little is known about the synergic impacts of geographic variation and anthropogenic pollution on MRGs. In this study, the abundance, composition and microbes of MRGs in four cascade reservoirs along the Jinsha River, southwestern China were investigated via high-throughput metagenomics. The results showed significant enrichment of chromium, cadmium and lead in Ludila and Xiluodu reservoirs with moderate ecological risks based on the criteria of drinking water quality and aquatic life protection. Nevertheless, at watershed scale, these metals played little role in up-regulating MRGs abundance owing to the limited toxic stress on microbes. Accordingly, geographic variation showed stronger impacts on MRGs composition than metals as revealed by the distance-decay relationship (Pearson correlation, rgeo = 0.24–0.57, rmetal = 0.10–0.41) and co-occurrence network (Node degree to MRGs subtype, ngeo = 180, nmetal = 6). River damming, as an artificial isolation of geographic space, significantly affected MRGs composition. The longer operation history, smaller storage capacity and higher regulation frequency caused the higher dissimilarity of MRGs composition between the reservoir's upstream and downstream areas. In conclusion, the metal pollution level is a prerequisite regulating MRGs; while under the lowly-polluted conditions, geographic variation had stronger impacts on MRGs than metal pollution via altered assembly of microbial communities. This study provides an important guidance for the future environmental management and ecological protection of river-reservoir ecosystems.